The leucine operon of Escherichia coli and Salmonella typhimurium is controlled by transcription attenuation mechanism. Hybridization experiments will be performed to determine whether this is the major mechanism of control. To further characterize the operon and its control, nucleotide sequence analysis will be carried out on regions of the operon and on mutants in which expression of the operon is altered. The position at which transcription starts in vitro will be determined. The function of an AT-rich region located upstream from the leu promoter will be investigated.